1. Field of the Invention
The present invention relates to a method of continuously forming a large area functional deposited film by sustaining a substantially uniform microwave plasma over a large area to cause plasma reactions by which a film forming raw material gas is excited and decomposed, and to an apparatus suitable for practicing the method.
Particularly, the present invention relates to a method capable of continuously forming a uniform functional deposited film over a large area while substantially improving the utilization and efficiency of a film-forming raw material gas at a high deposition rate and to an apparatus suitable for practicing the method. Specifically, the method and the apparatus enable mass-production of a large area thin film semiconductor device such as a photovoltaic device at a reduced cost.
2. Description of the Prior Art
As a result of marked generation of power in order to meet an increased demand in the recent years, serious problems of environmental pollution have arisen all over the world.
In fact, the system for generation of atomic power which had been anticipated as a power generation system capable of replacing the system for generation of steam-power and which has been in operation in some areas of the world, there have been occurrences where the systems were broken down causing radioactive contamination of living things including humans, as for example, the Chernobyl nuclear power plant accident. Because of this, there is fear of further development of the system for generation of atomic power. There are some countries that have already prohibited establishing any new atomic power plants.
In the case of steam-power generation, the amount of a fossil fuel represented by coal or petroleum consumed for power generation in order to satisfy the public demand for increased power supply has continuously increased. Accordingly, the amount of exhaust fumes from the steam-power generation plants has continuously increased so as to raise the content of gases, thereby causing a greenhouse effect such as accumulation of carbon dioxide gas in the air. This results in an earth-warming phenomenon. In fact, the annual average temperature of the earth has been heightened in recent years. In order to prevent the earth-warming phenomenon from further developing, the IEA (International Energy Agency) has proposed reducing the amount of carbon dioxide exhausted from the steam-power generation plant as much as 20% of the current level by the year 2005. Against this background, there is the situation that the populations of the developing countries will continue to increase and along with this, a demand for power supply will be increased. In addition, it is expected that the manner of living in the developed countries will be further modernized with further developments in electronic instruments and consequently, the amount of power consumption per person will be eventually increased. In view of the above, the matter of power supply is now a subject to be internationally discussed in terms of the earth.
In this situation, public attention has now focused on various studies that have been made on the power generation system using a solar cell since it has various advantages. It is a clean power generation system free of the foregoing problems relating to radioactive contamination, earth-warming and environmental pollution. The sunlight to be used as its energy source reaches everywhere on the earth and there is no problem of an energy source to be localized. Further, the power generation equipment can be simplified and a relatively high power generation efficiency can be attained. In order for the solar cell power generation system to be devised so that it can supply power in a quantity to satisfy the public demand, it is basically required that the solar cell be capable of providing a sufficiently high photoelectric conversion efficiency, can stably exhibit the solar cell characteristics and can be mass-produced.
In order to provide the average family with the power to meet consumption needs, a solar cell capable of outputting a power of about 3 KW is necessary. In this case, the photo-electric conversion efficiency of the solar cell should be about 10%. The solar cell is required to have an area of about 30 m.sup.2 in order to provide the power. In the case where it is intended to satisfy the demands for power supply of 100,000 families, the solar cell is required to have an area of 3,000,000 m.sup.2.
In view of this, public attention has been focused on an amorphous silicon solar cell which is prepared by depositing a semiconductor film such as an amorphous silicon semiconductor film on a relatively inexpensive substrate such as glass or metal sheet wherein glow charge is caused in a film-forming raw material gas such as silane gas. It is anticipated that it can be mass-produced and can be provided at a lower cost in comparison with a single crystal silicon solar cell. Various proposals have been made on the amorphous silicon solar cell.
In the case of the power generation system using a solar cell, there is usually a system employed in which a plurality of unit modules are connected in series or in parallel to form a unit from which a desired current or voltage can be obtained. For each of the plurality of modules, it is required that neither disconnection nor short circuit occur. It is further required that each of the plurality modules stably outputs an even current or voltage. In order to satisfy these requirements, each unit module must be prepared so that its constituent semiconductor layer, being an important element, be formed so as to stably exhibit uniform characteristics. Further, to simplify the design of the module and the process for assembling a plurality of unit modules to a unit, it is essential to provide a large area semiconductor film having uniformity not only in the thickness but also in quality and capability of exhibiting uniform semiconductor characteristics. These features enable economic mass production of a solar cell.
Now, in the solar cell, its constituent semiconductor layers, which are basically important constituent elements thereof, are conjugated to form a semiconductor junction such as pn junction or pin junction. These semiconductor junctions can be attained by respectively stacking different semiconductor layers having a different conduction type from one another, by ion-implanting or thermally diffusing a dopant of a different conduction type into one of the constituent semiconductor layers of the same conduction type.
This situation is to be more detailed in the case of the amorphous silicon solar cell. It is known that glow discharge is caused in a gas mixture composed of a film-forming raw material gas such as silane gas (SiH.sub.4) and a raw material gas capable of supplying an element to be a dopant such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) to form a semiconductor film having a desired conduction type. When a plurality of semiconductor films respectively having a different conduction type are formed successively on a substrate in this manner, these semiconductor films are conjugated to form a desired semiconductor junction. In view of this, various proposals have been made that respective constituent semiconductor layers are individually formed in the respective independent film-forming chambers and stacked on a substrate to form a desired semiconductor junction between each pair of the semiconductor layers stacked, thereby obtaining an amorphous silicon solar cell.
For instance, the specification of U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD (hereinafter called a "PCVD") apparatus wherein the so-called roll to roll system is employed. The continuous PCVD apparatus comprises a plurality of glow discharge regions, wherein a sufficiently long flexible substrate having a desired width is disposed in a route through which the substrate sequentially passes through a plurality of the glow discharge regions. The substrate is continuously conveyed in its longitudinal direction while forming the required conductive type semiconductor layer in each of the glow discharge regions. Thus, the desired devices having the semiconductor junction can be continuously formed. The specification describes a gas gate provided for preventing the diffusion and mixture of the dopant gas with the other glow discharge regions. The plurality of glow discharge regions are isolated one from the other by a slit-shape isolation passage way provided with means for forming a cleaning gas flow of Ar, H.sub.2, etc. It may be considered that this roll to roll PCVD apparatus will be suitable for the mass-production of a semiconductor device. However, this roll to roll PCVD apparatus is problematic in the case of mass-producing a semiconductor device with a plurality of semiconductor junctions since each of the constituent semiconductor layers is formed by the PCVD method using a RF energy, there is a limit for continuously forming those constituent semiconductor layers at a high deposition rate while maintaining the characteristics desired for each of those constituent semiconductor layers. That is, even in the case of forming a thin semiconductor layer of, for example, about 5000 .ANG., it is necessary to always sustain a substantially uniform plasma over a large area. However in this roll to roll PCVD apparatus, there are many correlated film-forming parameters which are difficult to be generalized and highly-skilled technicians are required to do so. In addition to this, there are still other unresolved problems for the roll to roll PCVD apparatus in that the decomposition rate and the utilization efficiency of a film-forming raw material gas are not sufficient making the product unavoidably costly.
Japanese Patent Laid-Open No. 61-288074 discloses an improved roll to roll film-forming apparatus comprising a reaction chamber containing a hollow shaped curtaining portion of a flexible substrate, the reaction chamber having a reaction space circumscribed by the hollow-shaped curtaining portions and the reaction chamber being provided with at least an activation chamber isolated from the reaction chamber. The film formation by this apparatus is carried out by introducing active species formed in the activation chamber and if necessary, other film-forming raw material gas into the reaction space, wherein they are chemically reacted with the action of heat energy to form a deposited film on the inner surface of the hollow-shaped curtaining portion positioned in the reaction chamber. This roll to roll film-forming apparatus is advantageous in the viewpoint that the apparatus can be relatively compact and the deposition rate of a film to be formed may be improved because of using active species in comparison with the known PCVD apparatus.
The film formation by this roll to roll film-forming apparatus utilizes the film-forming chemical reaction with the aid of heat energy. Therefore, when the deposition rate of a film to be formed is desired to be heightened, it is necessary to increase not only the flow rate of active species to be introduced into the reaction space but also the quantity of heat energy to be supplied thereinto. However, it is extremely difficult to do so since there is a limit not only for the manner of generating a large amount of the active species in the activation chamber and sufficiently introducing the active species into the reaction space at a high flow rate without leakage but also for uniformly supplying a large quantity of the heat energy into the reaction space.
In recent years, a PCVD method using microwave has been observed to exhibit various advantages which cannot be attained by the RF glow discharge decomposition method. That is, it is possible to heighten the energy density, to effectively generate a plasma and to maintain the plasma in a desired state.
For instance, the specifications of U.S. Pat. Nos. 4,517,223 and 4,504,518 describe processes for forming deposited thin film on a small area substrate in a microwave glow discharge plasma under a low pressure condition. These two patent specifications note that because the processes are conducted under the low pressure condition, any of these processes makes it possible to obtain a high quality deposited film at a remarkably high deposition rate while eliminating not only polymerization of active species which impacts negative effects to the characteristics of a film to be formed. However, neither of these two patent specifications mentions anything about uniform deposition of a film over a large area.
The specification of U.S. Pat. No. 4,729,341 discloses a low pressure microwave PCVD method and an apparatus suitable for practicing the same, wherein a photoconductive semiconductor thin film is deposited on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. However, the large area substrates are limited to cylindrical substrate. That is, the electrophotographic photoreceptors, and the teachings described therein are not directly applied to elongated substrates having a large area. Further, the film-forming process is to be practiced in a batch system and the amount of deposited film products obtained by one batch is limited. The specification does not teach anything about continuous film deposition on a large area planar substrate.
Hitherto, the microwave PCVD apparatus has encountered a problem of deterioration in reproducibility due to changes occuring during the time the microwave energy is introduced into the deposition film forming device because the deposited film undesirably adheres to the microwave introduction opening through which the microwave energy is introduced into the apparatus. Furthermore, the above-described microwave introduction opening must be periodically repaired, causing the deposited film manufacturing cost to be raised excessively.
Furthermore, the above-described adhesion of the deposited film to the microwave introduction opening raises the temperature of the deposited film simultaneously with the rise in the temperature of the microwave introduction opening due to the absorption heat of the microwave and the radiative heat of the plasma. In a case where the deposited film is thin, no critical problem takes place because the quantity of the absorbed microwave is not considerably large. However, the above-described quantity increases in proportion to the thickness of the deposited film. Thus, the temperature of the film deposited on the surface of the microwave introduction opening is raised considerably. As a result, an undesirable phase shift such as crystallization sometimes takes place. As a result, the absorption of the microwave by the deposited film on the microwave introduction opening can be greatly changed, causing the discharge state to be unstable. The worst of it is that the microwave introduction opening is broken. The breakage of the microwave introduction opening is a critical problem in manufacturing the deposited film.
Now, there are still various problems to be solved for large area film deposition by the MW-PCVD method because non-uniformity of a microwave energy is apt to occur in microwave plasma due to the wavelength of a microwave being short.
For instance, in this respect, there is an attempt to use a slow microwave structure in order to provide uniformity of the microwave energy. However, there is an inherent problem in the slow microwave structure in that there is very rapid fall off of microwave coupling into the plasma as a function of the transverse increase in the distance of the microwave applicator. In order to solve this problem, a proposal has been made that the spacing of the slow microwave circuit from a substrate to be processed be varied to thereby make the energy density at the surface of the substrate uniform along the direction of movement of the substrate. For example, such proposal can be found in the specification of U.S. Pat. No. 3,814,983 or the specification of U.S. Pat. No. 4,521,717. More particularly, the former patent specification discloses that it is necessary to incline the slow wave structure at a certain angle with respect to the substrate. However, the efficiency of transmitting the microwave energy with respect to the plasma is unsatisfactory. The latter patent specification discloses the use of two slow wave structures in an anti-parallel arrangement in a plane which runs parallel to the substrate. More particularly, the latter patent specification discloses that it is desired to set the two slow wave applicators at an angle to each other so that the planes normal to the medians of the applicators intersect at a straight line which extends parallel to the surfaces of the substrate to be processed and at right angles to the travel direction of the substrate. Additionally, in order to avoid structure interference between the two slow wave applicators, it is desired to displace the two s low wave applicators from each other transversely of the travel direction of the substrate by a distance equal to half of the space between the cross-bars of the waveguide. Several proposals have been made in order to provide plasma uniformity and more particularly, energy uniformity, as found, for example, in J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 126-130 and pp. 295-298. These reports describe a microwave reactor called a microwave plasma disc source (MPDS) and that the plasma is in the shape of a disc or tablet, with a diameter that is a function of microwave frequency. More particularly, the reports describe that the plasma disc source can be varied with the frequency of microwave. However, in the case of a microwave plasma disc source which is designed for operation at the normal microwave frequency of 2.45 GHz, the plasma confined diameter is at most about 10 centimeters and the plasma volume is at most about 118 cm.sup.3. Thus, this is far from being a large surface area. In the case of a system designed for operation at the lower frequency of 915 MHz, the lower frequency source would provide a plasma diameter of approximately 40 cm with a plasma volume of 2000 cm.sup.3 ; and the microwave plasma disc source can be scaled up to discharge diameters in excess of 1 m by operating at still lower frequencies, for example 400 MHz. However, it is extremely expensive to establish such an apparatus which can perform this.
In order to effectively provide high density plasma using microwave, means have been proposed to establish the electron cyclotron resonance condition (namely, the ECR condition) by arranging electro-magnets around the cavity resonator as found in Japanese Patent Laid-Open Publication Nos. 55-141729 and 57-133636. At academic meetings, etc., methods have been reported of forming various semiconductor thin films by utilizing high density plasma and some microwave ECR PCVD apparatus capable of performing such methods have been commercialized.
However, it has been generally recognized in the technical field to which the invention pertains that it is technically difficult to form a deposited film uniformly over a large area substrate because of non-uniformity of plasma caused by the wavelength of microwave and also because of non-uniformity of magnetic field distribution due to the use of the magnets for the control of plasma. In the case where the microwave ECR PCVD apparatus is intended to scale up so that film deposition over a large area can be done, there are various problems to be solved beforehand such as the necessity of scaling up the electric magnets providing means for preventing the apparatus from overheating and providing a special DC high power regulated supply.
Further, the deposited film obtained by the known microwave ECR PCVD method is usually inferior to the deposited film obtained by the known RF PCVD method with respect to the film property. Further, in the case of forming a deposited film on a substrate by the microwave ECR PCVD method, there is a distinguishable difference with respect to the film deposition rate and the film property between the film formed in the space where the ECR condition is established and the film formed in the space where the ECR condition is not established, that is in the dispersed magnetic field space. In view of this, the microwave ECR PCVD method is not suitable for the preparation of such a semiconductor device that is required to excel in quality and in uniformity with respect to the characteristics to be provided.
The foregoing U.S. Pat. Nos. 4,517,223 and 4,729,341 describe the necessity of maintaining very low pressure in order to provide high density plasmas. That is, they describe that the use of low pressure is necessary in order to obtain high film deposition rate and/or high gas utilization efficiency. However, none of the foregoing slow wave structure and electron cyclotron resonance methods is sufficiently capable of maintaining the relationship among high film deposition rate, high gas utilization efficiency, high power density and low pressure.
In view the above, there is an increased demand for eliminating the foregoing problems of the known microwave PCVD method and providing an improved microwave PCVD process which is free of such problems.
There are also other demands for providing a large area or lengthy thin semiconductor film excelling in quality, uniformity of characteristics and deposition rate which is desirably usable in not only solar cells but also in semiconductor device such as TFTs, photoelectric conversion elements for contact image sensors, switching elements, image input line sensors, etc.